1. Field of the Invention
The invention disclosed herein relates to pressure sensors, and in particular to low cost packaging of a pressure sensor with limited influence of surface charge accumulation in oil filled packages.
2. Description of the Related Art
Offset drift due to surface charge accumulation is a well-known phenomenon and common failure mode occurring in a wide variety of semiconductor devices. The failure mechanism involves device surface charge accumulation which drives formation of charge inversion layers. The inversion layers compromise otherwise electrically isolating junction states. Growth of the charge inversion layer permits parasitic current leakage through the epi-layer, resulting in sensing element offset drift. As with many other types of devices, pressure sensing elements are influenced by this phenomenon.
Present day designs for pressure sensing elements that include a field shield are susceptible to surface charge accumulation and exhibit severe offset drift due to sense element charging. This is especially the case when deployed in oil encapsulated package assemblies and applications.
In many package configurations, the pressure sensing element is encapsulated by a dielectric oil. The oil provides for coupling of external absolute or differential pressure inputs with the sense element. Unfortunately, this also serves to couple external, electrostatic charge residing on the package, or elsewhere, to the sensing surface of the pressure sensing element. Typically, charge coupling occurs through polar alignment of molecules in the oil in response to an external field, and associated space charge accumulation at an interface of the sense element and the oil. Consequently, comparatively large external static charge may be coupled to the sensing element via the molecular polarizability of the oil. Such charge may be residing on, for example, plastic housing assemblies used to package the sensing element or introduced to the housing by electrostatic discharge (ESD) to the plastic package. This high static charge is more than sufficient to cause severe output shift.
Further complicating performance is the volumetric expansion and contraction of the oil as temperature is increased or decreased. The oil expansion exerts pressure against the isolation diaphragm and causes the sensor output to vary.
Further complicating performance is leakage of oil. Generally, poor sealing of such oil filled packages has also led to degraded performance, and ultimately the demise of a pressure sensor.
Thus, what are needed are methods and apparatus to improve the performance of pressure sensors encapsulated in an oil containing package.